1. Field of the Invention
The invention pertains to the field of antenna alignment, and more particular to the alignment of an antenna with near field measurements.
2. Description of the Prior Art
Procedures for localizing the defective elements of the phased array antenna using near field measurements exist in the prior art. The underlining concept in all these methods is the utilization of the Fourier and inverse Fourier transform relationships between the peak element excitations and the array spectrum of the antenna (array factor). The array spectrum of a planar phased array antenna when multiplied by the element pattern determines the far field pattern of the array antenna. Since the element locations of the phased array antenna repeat with the same unit cell lattice in the aperture plane, the array spectrum is periodic in the spectrum space. Consequently, the whole array spectrum can be constructed with the spectrum of the fundamental period. Once the spectrum of the fundamental period in the spectrum space is known, the inverse Fourier transform of the array spectrum uniquely specifies the element excitations, thereby providing information for correcting individual element excitation errors and establish the antenna alignment. With these procedures, a full spectrum of the fundamental period cannot be acquired because the spectrum space is divided into visible and invisible regions, the visible region being within a unit circle defined by u.sup.2 +v.sup. 2 =1, centered at the center of the fundamental period, where u and v are propagating directions sin .theta. cos .phi. and sin .theta. sin .phi., respectively.
Many phased array antennas in which grating lobes are suppressed have element spacings that establish a spectrum space that extends beyond the visible region defined by the unit circle. The array spectrum of the fundamental period within the unit circle represents a propagating wave while the array spectrum of the fundamental period outside the unit circle represents an evanescent wave that decays exponentially as the distance from the antenna aperture increases. This evanescent wave cannot be directly measured. The visible array spectrum, however, can be determied by dividing the far field pattern by the element pattern. For the purpose of antenna alignment, the far field pattern must be specified both in amplitude and phase. Such a far field pattern can be provided with measurements in the near field of the antenna and extrapolating the measured data to the far field. Near field measurements rather than far field measurements are emphasized in an antenna alignment when the need exists for both the amplitude and phase characteristics of the far field pattern. The near field measurements are better suited for this purpose because the phase of the far field pattern may easily be referenced to the center of the antenna aperture.
Basic to the alignment procedure with near field measurements is access to the invisible spectrum of the fundamental period in the spectrum space. This problem arises whenever the fundamental period is not wholly within the visible space. Such a problem exists for wide angle scanning phased antennas. In these antennas, the elements are densely located in the aperture plane to prevent grating lobes from appearing in the visible space over an entire scan range. As the unit cell's size decreases, the area of the fundamental period in spectrum space increases, increasing the difficulty in obtaining the invisible spectrum. Because of this difficulty, some alignment methods of the prior art simply ignore the invisible spectrum and determine the element excitations applying the inverse Fourier transform to the visible spectrum only. Such an exclusion results in element excitation retrieving errors, which cannot be completely removed during the antenna alignment procedure. Errors caused by these prior art methods may be greater than those resulting from the truncation of the invisible spectrum when the far field pattern is determined by the Fourier transform of the aperture function, because the array spectrum close to the unit circle, even though it is visible, cannot be obtained accurately. To obtain the visible array spectrum, the far field pattern must be divided by the element pattern. It is not realistic to expect the element pattern in both amplitude and phase to be known accurately in the neighborhood of the unit circle since the amplitude at the boundary of the circle is zero. Consequently, the region of the spectrum space in which the array spectrum can be well defined is smaller than the visible space.
The above shortcomings of the alignment methods are overcome in the prior art by a spectrum merge technique disclosed in U.S. Pat. No. 4,453,164. This technique capitalizes on the inherent beam steering capability of a phased array antenna. The main beam is steered to a plurality of different directions, generally four, to achieve a desired accuracy, and near field measurements are performed at each beam position. When the four beam positions are utilized, a quarter of the fundamental period is brought within the visible space for each measurement. These fractional fundamental periods are then combined to establish a full spectrum over the fundamental period. The inverse Fourier transform of the constructed spectrum is then performed to establish individual element excitations from which excitation errors are determined. This method, however, is time consuming and does not provide sufficient accuracy for all applications.
One cause of inaccuracy of the spectrum merge method is the discontinuities that exist at the quadrant boundaries. These discontinuities are established by the difference in phase shifter and near field data acquisition errors encountered in the four near field measurements. Such discontinuities result in retrieved element excitation errors. Since the discontinuities stretch from the main lobe through the side lobes, the far-out side lobes as well as the side lobes near the main beam, which generally must meet stringent performance specifications, are effected.